Saturday, November 07, 2009

Rangkaian Multivibrator Monostable IC 555 is a pulse generator circuit in which the duration of the pulse is determined by the R-C connected to IC 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable (unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by an externally connected capaci tor C to a reference level. The time taken in storage determines the pulse width. The transition of output from stable state to quasi-stable state is accom­plished by external triggering.

Capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer it takes for the capacitor voltage to reach +2/3VCC. In other words, the RC time constant controls the width of the output pulse. The time during which the timer output remains high is given as

tp = 1.0986 RAC

where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the capacitor at any instant during charging period is given as

vc = VCC (1- e-t/RAC)

Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0 to +2/3VCC.

So +2/3VCC. = VCC. (1 – e-t/RAC) or t – RAC loge 3 = 1.0986 RAC

So pulse width, tP = 1.0986 RAC s 1.1 RAC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is widely used in industry for many different timing applications.